In recent years, silicon-containing polymers have become notable as functional materials such as heat-resistant materials, optical functional materials, ceramic materials and the like.
Inorganic fibers made of silicon-containing polymers developed to date have excellent heat resistance and dynamic properties compared to other materials, and are therefore employed for use under harsh conditions, for example, as aircraft materials and anti-pollution materials. As such inorganic fibers there have been proposed, for example, silicon carbide fibers (NIKALON™ by Nippon Carbon Co., Ltd.), silicon-titanium-carbon-oxygen fibers (Tirano Fibers™ by Ube Industries, Ltd.) and the like (see Patent document 1 and Non-patent document 1).
In particular, silicon-titanium-carbon-oxygen fibers are photocatalytically active and are used for decomposition of dioxins or for sterilization (see Non-patent document 1). However, because the fiber diameters of such fibers are limited to 5-8 μm, the surface area of fibers exhibiting photocatalytic activity has been insufficient. Moreover, fibers with a larger surface area are also preferred when these inorganic fibers are used as filters, and therefore thinner fibers have been desired.
Recently, nonwoven fabrics composed of inorganic fibers with photocatalytic properties have been proposed (see Patent documents 2 and 3).
However, the mean fiber diameter of fibers obtained by such production processes is about 5 μm, and production of inorganic fibers of 2 μm or smaller is very difficult by melt-blow production techniques. Also, since the obtained fibers are limited to smooth fibers it has not been possible to increase the area-to-weight ratio.
On the other hand, as a method of producing thin inorganic fibers there has been proposed a method in which an electric field is applied to a sol solution for spinning, and the filament is then sintered (Patent document 4). The fiber diameters obtained by this process are thinner than 2 μm, but it is very difficult to impart photocatalytic activity to the fibers by this production process. Moreover, the obtained fibers are essentially glass and have insufficient heat resistance.
[Patent document 1] JP-A H7-189039
[Patent document 2] JP-A 2004-60095
[Patent document 3] JP-A 2004-60096
[Patent document 4] JP-A 2003-73964
[Non-patent document 1] Ishikawa, H., “High-Strength Titanium Oxide Fibers”, Kogyo Zairyo, Nikkan Kogyo Shimbun, July 2002, Vol. 50, pp. 48-51.